This invention relates to a magnetic recording medium. More specifically, it relates to a perpendicular magnetic recording medium of which the coercive force is controlled to be suitably low and which has excellent perpendicular magnetic anisotropy, and a process for the production thereof.
A magnetic recording medium produced by the anodic oxidation of Al or Al alloy and the filling, by plating, of the fine pores of the resultant alumite with a ferromagnetic metal, such as Fe, etc., exhibits excellent perpendicular magnetic anisotropy owing to its large phase magnetic anisotropy, and such a magnetic recording medium is expected to be a high-density magnetic recording medium.
For example, when Al or Al alloy is anodized in an oxalic acid bath (bath voltage .about.40 V), the resultant alumite has a cell diameter of about 1,000 .ANG., and the 1,000 .ANG. cells therefore constitute minimum recording units. In a CoCr film, which is a typical perpendicular magnetization film, the diameter of columns within the film is 200 .about.300 .ANG., and its minimum recording unit is therefore considered to be .about.300 .ANG.. Hence, in order to achieve a high-density recording in the same level as in the CoCr film, a Fe plated alumite film with a small cell diameter is required. An alumite manufactured by using a sulfuric acid bath (bath voltage .about.17 V) has a small cell diameter of about 450 .ANG.. However, the diameter of a pore (pore formed in the cell) is about 150 .ANG., and when a perpendicular magnetization film is formed in that state by carrying out Fe plating to deposit Fe in the fine pores of the alumite, the problem is that the resultant film exhibits a perpendicular coercive force of 2,000 Oe or more, and the magnetic recording with a magnetic head is impossible.
Another existing problem is that when the coercive force is decreased to about .about.500 Oe by treating the anodic oxidation film to enlarge its fine pore diameter to about .about.350 .ANG., the shape magnetic anisotropy of the perpendicular magnetization film is not only degraded, but the interdistance of Fe particles is also shortened, and the magnetic separation of the Fe particles is insufficient. Thus, the magnetic film shifts from a perpendicular magnetization film to an in-plane magnetization film, and the perpendicular magnetic anisotropy necessary for a high-density recording medium is degraded. Further, in the above alumite produced by using an oxalic acid bath, its pore diameter is required to be enlarged to 400 .ANG. or more in order to achieve the corecive force of 1,000 Oe or less in which the magnetic recording is made possible.